Waves Flashcards
What is a progressive (moving) wave?
Something that carries energy from one place to another without transferring any matter
What is the frequency of a wave?
1/period
What is the wave equation that links the wave speed to the frequency and wavelength?
Wave Speed = Frequency x Wavelength
How to use an oscilloscope to find the speed of sound?
Use a signal generator to generate a frequency of 2-6khz
Adjust the dials on the oscilloscope until there is at least one complete cycle
Change the distance between the mic and the speaker so that the peaks of one wave
line up with the troughs of the other wave.
Measure the distance between the mic and the speaker
Calculate the frequency of the wave by measuring the period
Move the mic until the wave on the oscilloscope moves 1 wavelength and the troughs line up again.
The difference between the two distances is the wavelength
Repeat this process and find a mean wavelength and then calculate the wave speed using the formula
What are transverse waves?
e.g electromagnetic waves
The vibrations (oscillations) are perpendicular to the direction of the wave
What are longitudinal waves?
e.g sound waves
The direction of the vibrations(oscillations) is parallel to direction of the waves is travelling
What does it mean for a wave to be reflected?
When a wave hits a boundary and bounces back
What does it mean for a wave to be refracted?
The wave changes direction as it enters a different medium, due to it speeding up or slowing down.
What is intensity?
A measure of how much energy a wave is carrying.
I= P/A
What are the properties of EM waves?
They travel at 3x10^8 in a vacuum
They are all transverse
They can all be polarised (like all transverse waves)
Like all other progressive waves, they can carry energy
What does it mean to polarise a wave?
Forcing a wave to oscillate only in a single plane of direction. It can only happen in transverse waves
What is the relationship between polarization and reflection?
When light reflects it is partially polarised
What effect does reflection have on the intensity of light?
At certain angles, light is partially polarised, causing a change in it’s intensity
How can you use polarization to cause the intensity of a wave to be 0?
When there are two sources of polarization that are perpendicular to each other, the intensity is 0
What is an interface?
The boundary between 2 different meida
What happens to ultrasound waves when they hit an interface?
They are partially reflected and partially transmitted through the media
What are ultrasound waves?
Waves that carry a frequency beyond the 20khz human hearing range
What are ultrasound scans?
Short pulses of ultrasound radiation from images inside the body
What process is used for ultrasound scans?
Ultrasound is directed into the body using a transducer
A gel is used to remove air pockets between air and skin and increase the penetrative effect on the ultrasound
When the ultrasound meets the interface inside your body (between different types of tissue) some of the ultrasound is reflected
A computer times how long it takes for the wave to return and uses that information to calculate the distance
Using the information it’s calculated the computer can generate an image
How can you produce a clearer image from an ultrasound?
By having shorter pulses ( but not so short that they interfere with the transmission of the transducer)
Shorter wavelengths as they diffract less
What is superposition?
When two or more waves pass through each other
What is the principle of superposition?
When two or more waves pass over each other the resultant displacement is equal to the vector sum of the individual displacements
What is constructive interference?
When two displacements (crest and a crest) or (trough and trough) superpose and add together to give a bigger displacement
What is destructive interference?
When two or more displacements add together and cancel each other out.
What does it mean for a wave to be in phase?
If two or more waves are both at the same point in their wave cycle. they always have the same displacement and velocity
When waves have a phase difference of 0 or a multiple of 360 or 2pi radians
What does it mean for a wave to be in anti-phase?
Waves with odd multiples of 180 or pi radians
What does it mean for waves to be coherent?
When the waves have the same wavelength and frequency and a fixed phase difference
What is a key property of interference patterns?
The two sources must be coherent
What is the path difference?
The amount by which the path of one wave is longer than the path of another wave
What are the main properties of path difference?
At any point that is an equal distance from both sources, there will be constructive interference.
At any point where the path difference is a multiple of a whole number of wavelengths there will be constructive interference
At any point where the path difference is an odd multiple of a half wavelength you will get destructive interference due to the waves arriving out of phase
What causes constructive interference to occur? (formula relating it to path difference)
path difference = nλ
What causes destructive interference to occur? (formula relating it to path difference)
path difference = (2n+1)λ / 2
What is a standing wave?
The superposition of two progressive waves with the same wavelength in the opposite direction.
They transmit no energy
What are nodes?
Where the amplitude is 0
What are antinodes?
Points of maximum amplitude
What is the first harmonic?
1/2(wavelength) of a wave
How can you investigate the factors affecting the resonant frequencies?
Measure the mass length of the string
Use this to find the mass per unit length ( m/L)
Set up an apparatus using:
signal generator, vibration transducer, string, and pulley
Measure the tension using (T = Weight)
Turn on the signal generator and very the frequency until you find the first harmonic
Keep the string type and the tension in the string the same and alter the length. Do this by moving the transducer and then find the first harmonic again.
What are the properties of the resonant frequency?
The longer the string, the lower the resonant frequency
The heavier the string, the lower the resonant frequency
The looser the string the lower the resonant frequency
What is diffraction?
The way that waves spread out as they come through a narrow gap or round obstacles
What factor affects diffraction?
The size of the gap and the wavelength of the wave
What happens if the gap is large in comparison to the wavelength?
There would be very little diffraction
What happens if the gap is the same size as the wavelength?
The most difration
What happens if the gap is smaller than the wavelength?
The wave can no longer pass through or get reflected
What is Huygens’ construct?
The use of circles to predict how refraction will occur
What are sound waves
Longitudinal waves that oscillations are parallel to direction and has a series of rarefrations and compression
What are white light waves?
Summation of many waves of different frequency
How can diffraction cause changes in colours?
Diffraction causes waves to spread out, causes interpherence—> interpherence —> amplification and cancellation of certain wave lengths
What’s a diffraction grating?
A slide containing many equally spaced slits very close together.
The formula relating change in wavelength to the change in velocity of a wave?
v/c =Δλ/λ
Diffraction grating formula?
dsinθ = nλ
